Raman spectroscopy or imaging is a useful and powerful spectroscopic tool for chemical analysis of materials. Raman imaging relies on scattering of light, usually from a laser, preferably a monochromatic laser in the visible, near infrared, or near ultraviolet range. When the incident light utilized is monochromatic, the light reflected from the irritated area differs in wavelength from that of the light source, with the wavelength shift being utilized to determine the molecular composition of the material.
Raman spectroscopy is a fundamental tool used in materials science, biology, ceramics, pharmaceuticals, semi-conductors, energy, polymers, medicine, chemistry and physics. Raman spectroscopy can be utilized to analyze both dry (solid), gaseous and aqueous materials and requires little or no sample preparation. It provides information on the type of chemical bonds present in the analyte over a sample volume illuminated by a laser light. Included in the Raman spectrum is the information on the amount of chemical species, the macroscopic (crystallinity) and nanoscale morphology (molecular alignment) of the compound as well as its purity.
Raman spectroscopy can generate chemical images by collecting multiple observations (point by point, or line-scan mapping) over the sample space. It can take hours, sometimes a significant fraction of a day to generate such images. Therefore, Raman mapping is inapplicable for the analysis of dynamic systems or real-time imaging using available commercial systems. As an alternative to rastering the sample space, others have resorted to 2D-CCDs to acquire the Raman image in the wavenumber of interest (so called global Raman imaging). This has met with partial success; however, such systems are costly and they provide information over a limited field of view (less than 0.25 mm). Accordingly, existing global Raman imaging systems are cost-prohibitive, and they may still need rastering for larger field of views.
Users of Raman imaging are researchers in the academia, pharmaceutical industry, semiconductor industry, polymers, ceramics, forensic labs and art museums. To illustrate an application: active pharmaceutical ingredient (API) of drug tablets cannot be investigated expeditiously enough by Raman at the production line. Or, real-time monitoring of carbon nanotubes during synthesis is not feasible with Raman. Or, the build-up and loss of charged species in a lithium ion battery cannot be visualized over the full field of view. These are few examples where users have unmet needs.
In global Raman imaging, a tunable band-filter (dielectric or tunable liquid crystal filter) that passes only the wavenumber range of interest from the analyte allows the collection of Raman intensity distribution over each pixel of the 2D-CCD array over the entire field of view. In applications where the analyte information is obtainable from a single peak, the image can be acquired in a single acquisition sequence. In present versions of global Raman imaging, the excitation is applied to the sample via a single lens. This effectively illuminates less than 10% of the field of view at the center which in turn means that data are not collected from more than 90% percent of the sample space, see FIG. 1A for example. While the 2D CCD can gather information all around, limitation of the excitation to the central region limits data collection to the center.
In summary, various Raman imaging systems can have drawbacks including one or more of limited fields of view, relatively high cost to obtain, and long signal acquisition times. These limitations hinder the expansion of Raman imaging to real-time and/or high volume applications.